Lisa and Mario are the first non-celebrity couple to appear on the show
The first show of the ninth series of Big Brother has pulled in an audience of 5.4 million people.
Last year's launch show drew 6.2 million viewers. Thursday night's figures also included those watching the time-delayed Channel 4+1 service.
The opening show saw a record 16 contestants enter the house, including a blind comic and a "secret" couple.
The series' first twist will see one half of the couple "married" to another female contestant on Sunday night.
First into the house were couple Mario Marconi and Lisa Appleton, followed by students Luke Marsden and Stephanie McMichael.
The 16 housemates settle into the house
The four were called into the diary room and told Stephanie, 19, and Mario, 42, had to act as a couple, and keep the truth secret, or face eviction.
The housemates, who also include a blind part-time comedian, are competing to win the show's £100,000 top prize.
Mario, whose real name is Shaun, and Lisa, 40, from Warrington, are the first non-celebrity couple to appear in the Big Brother house.
They were greeted with a mixture of cheers and boos as they entered the house.
Student Luke says his friends think he looks like Justin Timberlake
Mario, it was revealed, once sold a Rolex watch to pay for breast enlargement surgery for Lisa.
Luke, 20, of Wigan, the third housemate to enter, was also booed by the waiting crowd.
Student Luke, who says his friends think he looks like Justin Timberlake, told Mario and Lisa on his arrival in the house that his entrance had been "bizarre" and "a blur".
Next in, wearing a short, black dress, was Stephanie, from Liverpool, who says she wants to be the most famous Big Brother contestant of all time.
The first four contestants were immediately called into the diary room by Big Brother and given the first task of the new series.
They were told that Mario and Stephanie had to pretend that they were a couple.
Big Brother said that if any other contestants found out that Mario and Lisa were in fact the real couple, the first four housemates would all face eviction.
The four were then told they had a few minutes to concoct a story before the next contestants entered the house.
Dale said he would "stab anyone in the back" to win
In a twist at the end of the show, presenter Davina McCall said Mario and Stephanie would be told they must get married on air.
What they would not be told was that the wedding would actually be a sham and not binding, she added.
The fifth person in the house was actress Rachel Rice, 24, of Torfaen, also wearing a black dress, who appeared in a film with Hugh Grant as a child.
She was followed in by Dale Howard, 21, a PE teaching student from Liverpool.
Big Brother viewers watched as he said he would "stab anyone in the back" to win the £100,000 prize money.
Christian sales assistant Sylvia Barrie, 21, from south London, was next in, followed by dance student Dennis McHugh, 23, from Edinburgh.
Blind radio producer Mikey Hughes, 33, from Ayrshire, was the ninth contestant to enter the house.
As he entered, he told fellow Scottish contestant Dennis: "The west is the best."
Alexandra says she is famous in her home town of Croydon
Single mother Alexandra De-Gale, 23, an accounts executive from Croydon, south London, was greeted by a chorus of boos.
She was followed into the house by executive chef Rex Newmark, 24, from London, who says he is "secretly shy but very confident".
Mohamed Mohamed, 23, who was born in Somalia but is now a toy demonstrator in London, whooped and hopped down the staircase as he entered the house before telling housemates he was shaking with nerves.
Vicky Pollard sound-alike Rebecca Shiner, 21, a kindergarten nurse, repeatedly shouted the name of her hometown Coventry to the crowd before she entered the house.
Ipswich-born Darnell Swallow, a 26-year-old albino who was raised in the US, was next into the house, ahead of part-time model Jennifer Clark, 22, from Durham.
The final contestant in the house was Kathreya Kasisopa, a 30-year-old massage therapist originally from Thailand but now living in Kent.
This year's Big Brother house boasts a luxury bedroom, as well as a dormitory-style room with short beds and scratchy blankets.
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